Next lecture: Induction/Signaling

• Requirements of inducer and responder cells

• Cascades of inductive events are involved in forming organs

• Examples of the kinds of cell communication

• Important signaling pathways in development– Notch, TGF, SHH, Wnt, FGF

“Official glossary” from Wolpert

• Induction: The process whereby one group of cells signals to another group of cells in the embryo and so affects their development

• Competence: The ability of a tissue to respond to an inducing signal. Embryonic tissues remain competent for a limited time (Can depend on prior inductive events)

Competence factors

• Not all tissues are able to respond to the same signals. Competence is actively acquired– Receptors, signaling molecule– Transcription factors (chromatin state)

• Competence factors are specifically required in the responding tissue, not the inducer– Demonstrated in recombination experiments

Recombination experiments: First name the players…(fig 6.1 from Gilbert)

Then play…Fig 6.3 Gilbert

Cascades of inductive events form the organ: Reciprocal induction

Of Gilbert

Two kinds of induction (plus one)

• Instructive interaction: A signal or factor which tells the developing cell what it is

• Permissive interaction: A signal or factor that allows the cell to become what it is

• Selective interaction. A factor allows the cell to be “selected” into one lineage or another after a stochastic (random) change

For example, CD4 vs. CD8 T cellsSelection vs. Instruction

TCR-I

TCR-II

CD8

CD4

Juxtacrine interactions

• Involve cell surface receptors on inducer and responding cells (no soluble factors)

• Cell death/apoptosis pathways– Fas (CD95)/FasL

• Notch/delta pathway– Involved in many binary cell fate decisions– Examples in flies, worms and mice

Cell death pathwaysFigure 6.27 of Gilbert

Notch is involved in a wide array of binary cell fate decisions

• C. elegans:Ventral uterine vs. Anchor cell and vulval development

• Drosophila: Neural vs. Epidermal cells

• Mouse: Embryonic lethal, Demonstrated affects in the immune system and others

• Human: Notch deficiencies cause birth defects

Notch/delta pathway(fig 6.29 of Gilbert)

This model is simplistic little evidence for nuclear localization of Notch in developing organisms

Players in the Notch pathway

• Receptor family: Notch/lin-12, glp-1

• Ligands: (DSL) Delta, Serrate, Lag2, Jagged

• Processing: ADAMs (Kuz, TACE) and Secretases (Sel-12/presenilins)

• Downstream effectors: CBF1, Su(Hairless), Lag1 (Collectively called CSL)

Proteolytic processing of Notch is

complicated Figure 1 from Weinmaster (2000)Curr. Opin. Genet. Dev. 10:363-369

Presenilins

• Multipass transmembrane proteins

• Mutated in inherited, early onset Alzheimer's disease

• Involved in cleavage of amyloid precursor protein (APP)

• Evidence that they are proteases or co-factors of a protease

Evidence linking presenilins with Notch signaling

• Homology between PS1 and Sel-12 of C.elegans (sel-12 facilitates Notch signaling)

• PS and Notch deficient animals have similar phenotypes (Drosophila and mice)

• PS is required for access of Notch to the nucleus and, thus, Notch signaling

• Defects can be rescued by providing exogenous PS

Current view of Notch signaling Figure 2 from Weinmaster (2000) Curr Opin Genet. Dev. 10:363-369

Notch involvement in cell fate

• Examples of Notch involvement in inductive interactions (signaling between non-equivalent cells)

• Lateral specification (occurs in a population of equivalent cells)– Involves the amplification of a stochastic small

difference between equivalent cells

Lateral specification in C. elegansFrom Greenwald (1998) Genes. Dev. 12:1751-62

Evidence for feedback mechanism in lateral specification

• Constitutive active mutants of lin12 have no AC

• Mutants eliminating lin12 activity have 2 AC

• Mixing experiments between two types above– Lin12 - cells always became AC– Lin12 WT cells always became VU

• Behavior different from WT in non-mosaic situation where these cells have equal chance to become AC or VU

One way to generate a bias in Notch activity-C. elegans vulval development

From Greenwald (1998) Genes. Dev. 12:1751-62

fate adoption is 2o-1o-2o

Gilbert Chapter 6 errors

• Lag2 is not secreted but transmembrane. This is the ligand for lin12/Notch (p.169)

• Figure 6.40 on T-lymphocyte signaling: The pathways leading from the receptors are not correct. (Notice, there is no reference)

• Figure 6.19: Details appear to be in dispute

Paracrine factors and interactions

• Transforming Growth Factor (TGF)-• Sonic Hedgehog

• Wnt

• Fibroblast Growth Factor (FGF)

• Retinoic Acid (RA)

TGF-

• Over 30 members of the TGF- family

• Subfamilies, TGF, Activin, BMP, Vg1

• Processed proteolytically with the C-terminal region conatining the mature peptide

• TGF peptides can homo- or hetero-dimerize

• TGF signal transduction involves multiple receptors which activate cytoplasmic “Smads”

Smad pathway (fig 6.20) p.159

Division of labor among SmadsCourtesy of J.F. Doody (J. Massague lab @MSKCC)

Smad 6 and 7 are inhibitory proteins induced by antagonists of the TGF signaling pathway, such as -interferon (Smad7)

The Smad familyFrom Piek, et. al. (1999) FASEB J. 13:2105-2124

A more complete TGF pathway From Piek, et. al. (1999) FASEB J. 13:2105-2124

Evidence for non-redundant TGF family involvement in development (due to expression patterns)

• TGF-1 knockout mice have defects in blood and vasculature (many die prenatally)

• TGF-2 knockout mice have multiple organ malformation (perinatal death)

• TGF-3 knockout mice die shortly after birth due to defects in pulmonary development

• Information from Piek, et. al. (1999) FASEB J.

Next lecture: Induction/Signaling

• Requirements of inducer and responder cells

• Cascades of inductive events are involved in forming organs

• Examples of the kinds of cell communication

• Important signaling pathways in development– Notch, TGF, SHH, Wnt, FGF